Gas handling for plastics liquefaction

ABSTRACT

The present invention relates to a method for removing high molecular weight high melting point hydrocarbon vapors from a hydrocarbon vapor offgas stream produced during the liquefaction of a solid waste plastic material to produce an oil that serves as a liquid feedstock for a partial oxidation reaction. The hydrocarbon vapor offgas stream (2) is directly contacted with a water spray (4) at a condensation temperature above the melting point of the high molecular weight hydrocarbons contained in the offgas. This results in the condensation and convenient removal of the high melting point hydrocarbons, referred to as &#34;waxes.&#34; One or more subsequent condensation steps can be conducted at lower condensation temperatures to remove the lower temperature condensable hydrocarbons. The remaining uncondensed vapors are then recycled to serve as a heater fuel for the liquefaction of the waste plastic material.

This application is a §371 filing of PCT/US97/12481 filed Jul. 14, 1997which is a continuation of U.S. application Ser. No. 08/887,978 filedJul. 3, 1997, now U.S. Pat. No. 5,837,037 which issued Nov. 17, 1998,and also claims the benefit of U.S. Provisional Application Ser. Nos.60/021,817 filed Jul. 16, 1996, now abandoned and 60/021,877 filed Jul.17, 1996, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for removing high molecular weightlow melting point hydrocarbon vapors from an offgas stream producedduring liquefaction of a waste plastic material, and more particularlyfor utilizing the offgas vapor stream as a heater fuel for theliquefaction process.

2. Description of the Prior Art

Diminishing natural resources as well as economic considerations haveled to the increasing use of organic feedstocks from impure sources,such as scrap or waste plastic materials.

Waste or scrap plastic materials often comprise at least one solidcarbonaceous thermoplastic and/or thermosetting material which may ormay not contain associated inorganic matter, such as fillers andreinforcement material. Such materials may be derived from obsoleteequipment, household containers, packaging, industrial sources,recycling centers and discarded automobiles. Scrap plastic comprisessolid organic polymers derived from sheets, films, extruded shapes,moldings, reinforced plastics, laminates and foamed plastics. Themixture of scrap plastics varies with the source and with the presenceof non-combustible inorganic matter incorporated in the plastic asfillers, catalysts, pigments and reinforcing agents.

It is desirable to convert particulate scrap plastic into a liquidhydrocarbonaceous feedstock for a partial oxidation reaction to producegas mixtures of hydrogen and carbon monoxide, referred to as synthesisgas, or simply "syngas." Syngas can be used to make other useful organiccompounds or as a fuel to produce power.

The partial oxidation reaction can be conducted in a free-flow unpackednoncatalytic quench gasifier. The reaction temperature varies about1800° F. to about 3000° F. and the reaction pressure is about 1 to about100 atmospheres, preferably about 30 to about 80 atmospheres.

SUMMARY OF THE INVENTION

The present invention relates to a method for removing high molecularweight high melting point hydrocarbon vapors from a hydrocarbon vaporoffgas stream produced during the liquefaction of a solid waste plasticmaterial to produce an oil that serves as a liquid feedstock for apartial oxidation reaction. The hydrocarbon vapor offgas stream isdirectly contacted with a water spray at a condensation temperatureabove the melting point of the high molecular weight hydrocarbonscontained in the offgas. This results in the condensation and convenientremoval of the high melting point hydrocarbons, referred to as "waxes."One or more subsequent condensation steps can be conducted at lowercondensation temperatures to remove the lower temperature condensiblehydrocarbons. The remaining uncondensed vapors are then recycled toserve as a heater fuel for the liquefaction of the waste plasticmaterial.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is a simplified diagrammatic representation ofthe offgas condensation operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Particulate waste plastic materials, even those containing halogens canbe converted by thermal cracking to an oil composition suitable as afeedstock for a partial oxidation reaction in a quench gasifier toproduce a synthesis gas.

The liquefaction of the particulate waste plastic materials,particularly bulk waste plastic materials involves melting the wasteplastic material by direct contact with a hot oil melting medium toproduce a molten viscous mixture of the waste plastic materials in thehot oil melting medium. The melting of the waste plastic material alsoproduces an offgas vapor which includes hydrocarbon vapors of varyingmolecular weights, carbon dioxide and water vapor. Depending upon thenature of the waste plastic material, acid halides and halohydrocarbonscan also be contained in the offgas vapor.

An important aspect of this invention is the treatment of the offgasesgenerated during the liquefaction of the particulate waste plasticmaterial to recover condensible hydrocarbons and to use uncondensedhydrocarbon vapors to fuel the heater used in the liquefaction of theparticulate scrap plastic materials.

Offgas vapors contain a mixture of condensible hydrocarbons of varyingmolecular weight, including high molecular weight hydrocarbons referredto as "waxes", which condense at temperatures on the order of about 210°F. to about 280° F. The offgas vapors also include lower molecularweight condensible hydrocarbons which condense at a temperature of about200° F., below which temperature the hydrocarbon waxes solidify.

Therefore, by exposing the hydrocarbon containing offgas from theliquefaction of waste plastics to a temperature below the melting pointof the hydrocarbon waxes can result in a mixture of condensed liquidhydrocarbons and solidified and/or highly viscous hydrocarbon waxes. Thesolidified waxes can cause serious plugging and fouling in thecondenser, as well as blockage problems in the gasification systempipelines and equipment.

It has been found that the initial condensation and separation of thehigh molecular weight hydrocarbon waxes from the offgas vapors at acondensation temperature above the melting point of the waxes, avoidsthe problem of blockage and plugging in the gasification systempipelines and equipment.

After the condensible waxes have been condensed and removed from theoffgas, the offgas temperature can then be further reduced to condenseand remove lower molecular weight condensible hydrocarbons in as manysubsequent cooling and condensation steps that are needed, dependingupon the composition of the offgas. The offgas treatment includes theremoval of water and any acid halide vapors, particularly hydrogenchloride (HCl) from the offgas.

Thus, the invention includes the removal of condensible hydrocarbons instages, depending upon the melting point of the hydrocarbons, so thathigh molecular weight "waxes" are removed from the offgas vapor prior tosubsequent hydrocarbon condensation at lower temperatures to removelower melting point condensible hydrocarbon vapors.

The invention can be more readily understood by referring to the FIGUREwherein an offgas hydrocarbon vapor stream 2 is the byproduct of themelting of particulate waste plastic materials in a hot oil liquefactionsystem to produce a molten viscous oil mixture and the offgas stream 2,which is directly contacted with water spray 4 to cool the offgas stream2 to a temperature of about 210° F. to 280° F.

The spray cooling of offgas stream 2 condenses high melting point, highmolecular weight hydrocarbon waxes at a temperature above the meltingpoint of the waxes, thereby liquefying but not crystallizing orsolidifying the waxes. Another purpose for the water spray, which can bein the form of an aqueous mist, is to attenuate the temperaturefluctuations of the offgas to produce a mixture of water, uncondensedhydrocarbon vapors and condensed hydrocarbon wax stream 6 which enterscondensate receiver 8, maintained at a temperature of about 210° F. to280° F.

The water spray 4 is preferably supplied from an ammonia rich waterstream exiting from an ammonia stripper (not shown) that is employed totreat scrubbing water that has been used as a scrubbing medium forsynthesis gas exiting a quench gasifier (not shown).

The condensed hydrocarbon waxes are separated from the remaininguncondensed offgas vapor and exit the condensate receiver 8 in stream 10and enter a second condensate receiver 12 that is maintained at atemperature of about 60° F. to about 140° F. The first condensatereceiver 8 can be physically located above the second condensatereceiver 12 so that the condensed liquid hydrocarbon wax stream 10 canflow by gravity from the receiver 8 to the receiver 12.

Uncondensed vapor stream 14, freed of the high molecular weighthydrocarbon waxes exits the condensate receiver 8 at a temperature ofabout 80° F. to about 140° F., and contains a mixture of hydrocarbons,water, carbon dioxide, and acid halides. As vapor stream 14 passesthrough the heat exchanger 16, additional hydrocarbon vapors condense toform a mixture with the remaining uncondensed vapors and exit as stream18 which then enters the second condensate receiver 12 that ismaintained at a temperature of about 60° F. to about 140° F. In thereceiver 12, substantially wax-free hydrocarbons, and most polar speciessuch as water, hydrogen halides, alcohols, glycols, aldehydes, organicacids, esters, and the like from stream 18 are separated from theremaining uncondensed hydrocarbon vapor and are combined with the highermolecular weight condensate wax stream 10, to form a combined condensatewhich exits condensate receiver 12 as stream 20.

An uncondensed vapor stream 22 is separated from stream 18 and exitscondensate receiver 12 by passing through a scrubbing tower 24 which canbe mounted directly on top of the condensate receiver 12. A caustic orammonium hydroxide scrubbing solution can be supplied to the scrubber 24to contact the vapor stream 22 and remove any traces of acid halidessuch as hydrogen chloride and to react with any chloromethane that mayalso be present in vapor 22 to form methanol which is returned toreceiver 12. Excess scrubbing solution from scrubber 24 can also flowback directly into condensate receiver 12.

The uncondensed vapor stream 22 exiting the scrubber 24 is cooled in anindirect heat exchanger 26 to a temperature of about 40° F. to about 80°F. Additional more volatile substances condense from vapor stream 22 toform condensate stream 28 comprising principally organic compoundscontaining 4 to 10 carbon atoms and water which exits the heat exchanger26 to combine with the condensed stream 20 that exits condensatereceiver 12 to form combined stream 30 which enters pump 32 whichperiodically discharges the condensate to storage or for use as achemical feedstock or as part of the feed to a gasification process. Thecooled uncondensed hydrocarbon vapor stream 34 exits heat exchanger 26and enters heat exchanger 36 where it is further cooled to a temperatureof about 10° F. to about 50° F., and wherein stream 38 condenses andcomprises principally hydrocarbon and halohydrocarbons containing 2-5carbon atoms, and enters the condensate receiver 12. Optionally, all ora portion of stream 38 can be combined with stream 30 and dischargedthrough pump 32 as noted above.

The remaining cooled uncondensed hydrocarbon vapor stream 40 exits heatexchanger 36 at a temperature of about 10° F. to about 50° F., entersheat exchanger 42 and exits as cooled hydrocarbon vapor stream 44 at atemperature of about -40° F. to about 10° F. Vapor stream 44 optionallyenters the absorber 46 to remove any remaining traces of organichalides, and exits as hydrocarbon vapor stream 48 which is then recycledthrough the heat exchanger 36 as the cooling medium, and exits as warmedhydrocarbon vapor stream 50 at a temperature of about 20° F. to 60° F.,to serve as a fuel for the liquefaction heater which melts theparticulate waste plastic materials during the waste plasticliquefaction operation (not shown).

What is claimed is:
 1. A method for removing high molecular weight, highmelting point hydrocarbon vapors by condensation from ahydrocarbon-containing offgas vapor produced during the liquefaction ofparticulate waste plastic material, comprising:(a) contacting thehydrocarbon-containig offgas vapor directly with water at a condensationtemperature above the melting point of the high molecular weighthydrocarbon vapors to produce a first high molecular weight liquidhydrocarbon condensate and a first uncondensed vapor stream; (b)separating the first high molecular weight liquid hydrocarbon condensatefrom the first uncondensed vapor stream; (c) cooling the firstuncondensed vapor stream to a temperature of about 180° F. to about 200°F. to produce a second liquid condensate and a second uncondensed vaporstream; (d) separating the second liquid condensate from the seconduncondensed vapor stream; and (e) contacting the second uncondensedvapor stream with a caustic scrubbing solution to neutralize any halidevapors and to form a hydrogen halide acid-free vapor stream.
 2. Themethod of claim 1, wherein the first hydrocarbon condensate and thesecond hydrocarbon condensate are combined to form a single hydrocarboncondensate.
 3. The method of claim 1 (a), wherein the water used tocontact the hydrocarbon containing offgas vapor stream is in the form ofa spray.
 4. The method of claim 3, wherein the water contains ammonia orcaustic.
 5. The method of claim 3, wherein the water is supplied from anammonia rich water stream exiting an ammonia stripper.
 6. The method ofclaim 5, wherein the ammonia stripper is used to treat synthesis gasscrubbing water.
 7. The method of claim 1 (a) wherein the water is at atemperature of about 210° F. to about 280° F.
 8. A method for preventingblockage and plugging of piping and equipment by hydrocarbon waxes thatare condensed from a hydrocarbon containing offgas from the liquefactionof waste plastic materials, comprising:(a) contacting thehydrocarbon-containing offgas vapor directly with water at acondensation temperature above the melting point of the high molecularweight hydrocarbon vapors to produce a first high molecular weightliquid hydrocarbon condensate and a first uncondensed hydrocarbon vaporstream; and (b) separating the first high molecular weight liquidhydrocarbon condensate from the first uncondensed hydrocarbon vaporstream.